This invention relates to a mechanical fixture enabling an accurate T-peel test of thin and flexible adhered laminates.
A peel test is a simple and popular mechanical test for measuring adhesion strength in a variety of applications, particularly for use with bonded thin and flexible laminates, and carried out in several different ways. The geometry usually consists of a laminate bonded to another laminate or to a thick substrate and the test is conducted by pulling the laminate off the other laminate or substrate at some angle, while recording the peeling-force in the steady-state debonding. The usual goal is to relate the experimentally obtained peel force to the intrinsic toughness of the interface, where the toughness of the interface represents the work required per unit area to advance a crack at the interface and has the units of J/m2 or N/m. Only in very restricted scenarios can the peel force give a direct estimate of interface toughness. More generally, the peel force is affected by the geometry of the samples, the constitutive properties of the laminates, inter-facial properties, etc.
Depending upon the application, several existing ASTM standards such as ASTM D903-98(2010). ASTM D3807-98(2012), ASTM D6252, ASTM D1876-08, ASTM D3330, ASTM D6862, ASTM F88/F88M-09 or ISO standards ISO 11339:2010, ISO 8510-2:2006, ISO 14676:1997, ISO 29862:2007 are commonly employed for measurement of adhesion through peeling. These standards can be easily practiced on various commercially available fixtures, for e.g. 90 Degree, 180 Degree, Climbing Drum, Floating Roller, Adjustable Angle, German Rotating Wheel, etc. peel fixtures. The present invention deals with a particular kind of peel test, known as T-peel test (T indicating that the specimen forms a T-like shape between symmetrical interfaces).
The T-peel test is quite straightforward to perform: the unbonded parts of two flexible laminates are clamped in the grips of a mechanical tester and separated apart. If the test is performed on a vertical mechanical tester, the bottom grip is usually held fixed while the top grip moves upwards. On vertical test machines, when T-peel specimens are thin and flexible, the action of gravity can lead to an asymmetric configuration. In this situation, the bending of the freely suspended end (hangover tail) and the degree of anti-symmetry introduced depends upon the geometry (length, width and thickness) and the material properties (density, modulus, etc.) of the specimens.
In critical scenarios, the bending action of gravity can plausibly induce plastic deformation in the lower peel arm in addition to a mixed-mode failure. All these effects can lead to a deviation in the measured peel force compared to an ideal and symmetrical T-peel. The uncertainty in the asymmetry during such a test is uncontrolled, degree of mode-mixity (or phase angle) is unknown, and thus no straightforward correction is possible. These issues lead to an uncertain estimation of inter-facial fracture toughness. In recent years, there have been some studies on the role of asymmetry in T-peel specimens, and a proposal of a testing apparatus to perform the T-peel in a layout such that the tail of the specimen aligns with gravity. However, in this configuration undesired twisting and bending of the T-peel specimen also occurs. In some cases the recommendation to compensate for the tail hangover during a peel-test is through hand-support. It is quite obvious that any such manual endeavor is not robust.
A principal object of this invention, therefore, is to provide a new and improved fixture and technique for a T-peel test that eliminates the limitations of prior T-peel test methods.
A further object of this invention is to provide a novel design of a mechanical fixture to support a freely suspended end of a T-peel specimen and thereby suppress the effect of gravity.